Musk's mistake, as others mention, was positioning himself as a free speech absolutist, which was a promise he could not possibly keep, nor did he really want to. He's following the same sort of approach as other tech CEOs.
edit: the parent comment has been edited since my reply, it used to make a bit more sense.
> The Russian "Firehose of Falsehood" Propaganda Model
> Why It Might Work and Options to Counter It
Imagine you're setting a standard on free speech and you draw the line at the mention of Mastodon.
The guy who runs GAB, pretty much everything that's not pornography is allowed there.